A high-temperature, high-strength Co—Ni base alloy and a method of making an article therefrom are disclosed. More particularly, a gamma prime (γ′) strengthened Co—Ni base alloy that is capable of forming a protective, adherent oxide surface layer or scale is disclosed together with a process for producing the same. These alloys are suitable for making articles for applications where high temperature strength and oxidation resistance are required.
In a number of high-temperature applications, particularly for use in industrial gas turbines, as well as engine members for aircraft, chemical plant materials, engine members for automobile such as turbocharger rotors, high temperature furnace materials and the like, high strength is needed under a high temperature operating environment, as well as excellent oxidation resistance. In some of these applications, Ni-base superalloys and Co-base alloys have been used. These include Ni-base superalloys which are strengthened by the formation of a γ′ phase having an ordered face-centered cubic L12 structure: Ni3(Al,Ti), for example. It is preferable that the γ′ phase is used to strengthen these materials because it has an inverse temperature dependence in which the strength increases together with the operating temperature.
In high-temperature applications where corrosion resistance and ductility are required, Co-base alloys are commonly used alloys rather than the Ni-base alloys. The Co-base alloys are strengthened with M23C6 or MC type carbides, including Co3Ti, Co3Ta, etc. These have been reported to have the same L12-type structure as the crystal structure of the γ′ phase of the Ni-base alloys. However, Co3Ti and Co3Ta have a low stability at high temperature. Thus, even with optimization of the alloy constituents these alloys have an upper limit of the operating temperature of only about 750° C., which is generally lower than the γ′ strengthened Ni-base alloys.
A Co-base alloy that has an intermetallic compound of the L12 type [Co3(Al,W)] dispersed and precipitated therein, where part of the Co may be replaced with Ni, Ir, Fe, Cr, Re, or Ru, while part of the Al and W may be replaced with Ni, Ti, Nb, Zr, V, Ta or Hf, has been disclosed in US2008/0185078. Under typical oxidation conditions, the Co-base alloys strengthened with Co3(Al,W) typically form cobalt-rich oxides, such as CoO, Co3O4 and CoWO4, which are not protective and result in poor oxidation and corrosion resistance. While good high-temperature strength and microstructure stability have been reported for this alloy, further improvement of the high-temperature properties are desirable, including high-temperature oxidation and corrosion resistance, particularly high-temperature oxidation resistance.